1. Technical Field of the Invention
The present invention relates to an echo sound signal suppressing apparatus for and an echo sound signal suppressing method of suppressing an echo sound signal to ensure an echo-reduced interactive speech communication among far-end speakers and near-end speakers in at least two conference rooms distant from each other and each having therein a hand-free speakerphone provided to enable both the speakers to speak and listen with no handset microphone-speaker unit, and more particularly to an echo sound signal suppressing apparatus and an echo sound signal suppressing method which can enhance the quality of a second sound from the near-end speakers to be received by the far-end speakers by discriminating the second sound signal component from the echo sound signal component and the outside noise signal generated in the neighborhood of the near-end speakers.
2. Description of the Related Art
Up until now, there have been proposed a wide variety of echo sound signal suppressing apparatuses of this type one typical example of which is shown in FIG. 29. The conventional echo sound signal suppressing apparatus used for the interactive speech communication between the far-end speakers and the near-end speakers will be described hereinafter with reference to only two speakers including a far-end speaker and a near-end speaker for brevity. The conventional echo sound signal suppressing apparatus comprises a first amplifier 4, a loudspeaker 1, a microphone 2, a second amplifier 5, an input terminal 10, and an output terminal 11.
The first amplifier 4 is designed to amplify a first sound signal from the far-end speaker. The first sound signal comprises a far-end speaker sound signal component indicative of the far-end speaker sound from far-end speaker and a first noise signal component indicative of the far-end outside noises generated in the neighborhood of the far-end speaker, the first sound signal having a power value, and the first noise signal component having a power value. The loudspeaker 1 is adapted to output a first sound in its loudened state after converting the first sound signal from the far-end speaker to the first sound, the loudspeaker 1 having an input terminal connected to a communication line to have the first sound signal pass therethrough.
The microphone 2 is operative to input three different sounds before converting the three different sounds to a second sound signal. The three different sounds comprises a second sound from the near-end speaker, an echo sound indicative of the first sound reflected under the influence of the objects outside the near-end speaker, and near-end outside noises generated in the neighborhood of the near-end speaker. The objects include attendants taking part in the conference, and other bodies serving to reflect sounds. The second sound signal comprises a near-end speaker sound signal component indicative of the second sound from the near-end speaker, an echo sound signal component indicative of the echo sound, and a second noise signal component indicative of the near-end outside noises generated in the neighborhood of the near-end speaker. The second sound signal has a power value, and the second noise signal component has a power value. The second amplifier 5 is designed to amplify the second sound signal received by the microphone 2. The conventional echo sound signal suppressing apparatus further comprises echo replica sound signal producing means 8b, difference signal producing means 8c, attenuation level calculating means 7, and echo sound suppressing means 6. The echo replica sound signal producing means 8b is designed to produce an echo replica sound signal based on both the first sound signal and the second sound signal and having a power value. The difference signal producing means 8c is operative to produce a difference signal by subtracting the echo replica sound signal from the second sound signal. The difference signal comprises the near-end speaker sound signal component and a difference signal component indicative of subtracting the echo replica sound signal from the second sound signal. The difference signal component has a power value, and the difference signal has a power value.
The attenuation level calculating means 7 is designed to divide three different information comprising the first sound signal, the second sound signal, and the difference signal into a plurality of frames along the time axis to calculate three different power values of the sequential frame comprising a power value of the first sound signal of the sequential frame, a power value of the second sound signal of the sequential frame, and a power value of the difference signal of the sequential frame before calculating an evaluation value of the power value of the difference signal of the sequential frame. The evaluation value of the power value of the difference signal is calculated by the following equation (1),                               EC          ⁡                      (            n            )                          =                  10          ⁢                      log            10                    ⁢                                    ∑                              i                =                0                                            L                -                1                                      ⁢                          xe2x80x83                        ⁢                                                            e                  ⁡                                      (                                          n                      -                      i                                        )                                                  2                            /                                                ∑                                      i                    =                    0                                                        L                    -                    1                                                  ⁢                                  xe2x80x83                                ⁢                                                      y                    ⁡                                          (                                              n                        -                        i                                            )                                                        2                                                                                        (        1        )            
wherein the legend xe2x80x9cEC(n)xe2x80x9d is indicative of the evaluation value of the power value of the difference signal of the sequential frame, the legend xe2x80x9ce(nxe2x88x92i)xe2x80x9d is indicative of the difference signal, and the legend xe2x80x9cy(nxe2x88x92i)xe2x80x9d is indicative of the second sound signal.
The attenuation level calculating means 7 is also designed to compare the power value of the difference signal of the current frame with the power value of the difference signal of the preceding frame to judge whether or not the sequences of the evaluation value of the power value of the difference signal for each of the sequential frame tend to a convergence value. The attenuation level calculating means 7 further serves to calculate an attenuation level sufficient to suppress the echo sound signal based on the four different information comprising the power value of the first sound signal, the power value of the second sound signal, the power value of the difference signal, and the convergence value when the attenuation level calculating means 7 is designed to judge whether or not the convergence value of the power value of the difference signal of the sequential frame exceeds a predetermined threshold value while the attenuation level calculating means 7 is designed to judge whether or not the power value of the far-end speaker sound signal exceeds a predetermined threshold power value.
The echo sound suppressing means 6 is operative to suppress the echo sound signal to output the near-end speaker sound signal with the suppressed echo sound signal component based on the attenuation value after receiving the attenuation value from the attenuation level calculating means 7. The echo sound signal not suppressed leads to the fact that the conventional echo sound signal suppressing apparatus tends to output the second sound signal with the echo sound signal to the far-end speaker when the attenuation level calculating means 7 is designed to make judgments as the power value of the difference signal exceeds a predetermined threshold value under the condition that the echo sound indicative of the first sound reflected under the influence of the fluctuation of the objects outside of the near-end speaker is received by the microphone, while the near-end speaker does not speak.
The second sound signal inputted from the microphone 2 to the attenuation level calculating means 7 through the amplifier 5 contains a far-end sound signal component, an echo sound signal component, and a second noise signal component mixed to one another. The near-end sound signal component of the second sound signal thus mixed cannot be discriminated by the attenuation level calculating means 7 from the echo sound signal component and the second noise signal component, thereby making it impossible for the echo sound signal suppressing apparatus to suppress only the echo sound signal component and the noise signal component to a minimum level. The echo sound signal component thus not suppressed results in deteriorating the quality of the second sound of the near-end speaker during the interactive speech communication between the far-end speaker and the near-end speaker. In other words, the echo sound signal suppressing apparatus makes it impossible for the far-end speaker to receive a clear sound from the near-end speaker without echo and outside noises generated in the neighborhood of the near-end speaker.
It is, therefore, an object of the present invention to provide an echo sound signal suppressing apparatus which can enhance the quality of the second sound from the near-end speaker to be received by the far-end speaker by discriminating the second sound signal component from the echo sound signal component and the outside noise signal generated in the neighborhood of the near-end speaker.
It is another object of the present invention to provide an echo sound signal suppressing apparatus which can suppress the echo sound signal component and the outside noises signal to a minimum level to realize echo and noises reduced interactive speech communication between the far-end speaker and the near-end speaker without the echo and noises and with the clear sound received by the far-end speaker.
The first aspect of the echo sound signal suppressing apparatus according to present invention comprising: a loudspeaker for outputting a first sound in its loudened state after converting a first sound signal from the far-end speaker to the first sound, the loudspeaker having an input terminal connected to a communication line to have the first sound signal pass therethrough, the first sound signal comprising a far-end speaker sound signal component indicative of a first sound from the far-end speaker, and a first noise signal component indicative of far-end outside noises generated in the neighborhood of the far-end speaker, the first noise signal component having a power value; a microphone for inputting three different sounds before converting the three different sounds to a second sound signal, the three different sounds comprising a second sound from the near-end speaker, an echo sound indicative of the first sound reflected under the influence of the objects outside the near-end speaker, and near-end outside noises generated in the neighborhood of the near-end speaker, the second sound signal comprising a near-end speaker sound signal component indicative of the second sound from the near-end speaker, an echo sound signal component indicative of the echo sound, and a second noise signal component indicative of the near-end outside noises generated in the neighborhood of the near-end speaker, the second sound signal having a power value, and the second noise signal component having a power value; echo replica sound signal producing means for producing an echo replica sound signal with the first sound signal and the second sound signal, the echo replica sound signal having a power value; difference signal producing means for producing a difference signal by subtracting the echo replica sound signal from the second sound signal, the difference signal comprising the near-end speaker sound signal component and a difference signal component indicative of subtracting the echo replica sound signal from the second sound signal, and having a power value; first power value calculating means for calculating the power value of the second sound signal; second power value calculating means for calculating the power value of the echo replica sound signal produced by the echo replica sound signal producing means; echo sound suppressing means for suppressing the echo sound difference signal component outputted from the difference signal producing means to a minimum level with the power value of the second sound signal calculated by the first power value calculating means and the power value of the echo replica sound signal calculated by second power value calculating means with the near-end speaker sound signal component being allowed to pass therethrough, the echo sound suppressing means having an output terminal to the communication line to output the near-end speaker sound signal component under the condition that the echo sound signal component is reduced to its smallest level.
The second aspect of the echo sound signal suppressing apparatus according to present invention comprising: first filtering means for filtering the second sound signal to be discharged to the first power value calculating means as a band-limited second sound signal indicative of the second sound having a frequency bandwidth limited to a predetermined frequency range; and second filtering means for filtering the echo replica signal to be discharged to the second power value calculating means as a band-limited echo replica signal indicative of the echo replica signal having a frequency bandwidth limited to a predetermined frequency range; and in which the first power value calculating means is operative to calculate the power value of the band-limited second sound signal of the sequential frame after receiving the band-limited second sound signal from the first filtering means; and the second power value calculating means is operative to calculate the power values of the band-limited echo replica sound signals of the sequential frames after receiving the band-limited echo replica sound signal from the second filtering means, echo sound suppressing means for suppressing the echo sound difference signal component outputted from the difference signal producing means to a minimum level with the power value of the band-limited second sound signal calculated by the first power value calculating means and the power value of the band-limited echo replica sound signal calculated by second power value calculating means with the near-end speaker sound signal component being allowed to pass therethrough, the echo sound suppressing means having an output terminal to the communication line to output the near-end speaker sound signal component under the condition that the echo sound signal component is reduced to its smallest level.
The third aspect of the echo sound signal suppressing apparatus according to present invention comprising: first sound signal dividing means for dividing the first sound signal into a plurality of sub-band section along a frequency axis of the first sound signal, the first sound signal of the sub-band section comprising a far-end speaker sound signal component of the sub-band section and a first noise signal component of the sub-band section; second sound signal dividing means for dividing the second sound signal into a plurality of the sub-band section along the frequency axis of the second sound signal, the second sound signal of the sub-band section comprising a near-end speaker sound signal component of the sub-band section, an echo sound signal component of the sub-band section, and a second noise signal component of the sub-band section, the second noise signal component of the sub-band section having a power value of the sub-band section; and difference signal synthesizing means for synthesizing the difference signal after receiving the difference signals of the sub-band sections; and in which the echo replica sound signal producing means is operative to produce an echo replica sound signal of the sub-band section with the first sound signal of the sub-band section and the second sound signal of the sub-band section, the echo replica sound signal of the sub-band section having a power value of the sub-band section; the difference signal producing means is operative to produce a difference signal of the sub-band section by subtracting the echo replica sound signal of the sub-band section from the second sound signal of the sub-band section, the difference signal of the sub-band section comprising a near-end speaker sound signal component of the sub-band section, a difference signal component of sub-band section indicative of subtracting the echo replica sound signal of the sub-band section from the second sound signal of the sub-band section, and the second noise signal component of the sub-band section, the difference signal of the sub-band section having a power value of the sub-band section; the first power value calculating means is operative to calculate the power value of the second sound signal of the sub-band section; the second power value calculating means is operative to calculate the power value of the echo replica sound signal of the sub-band section; the echo sound suppressing means is operative to suppress the difference signal component of the sub-band section outputted from the difference signal producing means to a minimum level of the sub-band section with the power value of the second sound signal of the sub-band section calculated by the first power value calculating means and the power value of the echo replica sound signal of the sub-band section calculated by the second power value calculating means, the echo sound suppressing means having an output terminal to the communication line to output the near-end speaker sound signal component of the sub-band section under the condition that the echo sound signal component of the sub-band section is reduced to its smallest level of the sub-band section.